A critical hurdle in realizing rechargeable lithium-metal batteries is the dendritic electrodeposition of lithium during the battery charging process. Here, we investigate the onset of dendritic morphology during galvanostatic lithium electrodeposition using electrochemical techniques combined with optical microscopy. We show that lithium dendrites initiate at the time when the surface overpotential during galvanostatic electrodeposition reaches a maximum value. This observation is explained using an analytical transport model wherein the Li+ concentration within the solid electrolyte interphase (SEI) near the lithium metal surface decreases gradually as the SEI thickness and its transport resistance increases with time. At the dendrite onset time (tau(onset)), the Li+ concentration at the lithium-SEI interface approaches zero - a condition under which surface roughness on the lithium electrode amplifies, producing dendrites. Once dendrites form, they rupture the SEI, lowering the surface resistance for plating. Model predictions of how t onset varies with current density and soak time are shown to be in qualitative agreement with experimental observations. Furthermore, pulsed currents are applied to mitigate Li+ concentration depletion within the SEI, thereby delaying or preventing altogether the formation of lithium dendrites. (C) The Author(s) 2018. Published by ECS.